The present invention relates to a distributed feedback semiconductor laser, and more specifically to a distributed feedback semiconductor laser having an excellent single mode operation stability in a wide temperature range.
A distributed feedback semiconductor laser (DFB semiconductor laser) is used as a light source for a long-distance, large-capacity optical communication. Recently, it has been studied to use the DFB semiconductor laser for a relatively short distance communication. In this application, it is necessary to reduce the cost of a DFB semiconductor laser module, and therefore, a demand for a DFB semiconductor laser operating without a temperature maintaining instrument which was incorporated in a prior art DFB semiconductor laser module, is increasing.
For this purpose, a DFB semiconductor laser having an active layer of a strained multiple quantum well structure has been developed, which has a high-temperature operation characteristics more excellent than that of a semiconductor laser having a prior art bulk active layer.
In the DFB semiconductor laser having the active layer of the strained multiple quantum well structure, however, a gain spectrum has a large temperature dependency. In addition, when the DFB semiconductor laser is operated at a high temperature or at a low temperature, a single mode oscillation does not occur. The reason for this is considered that for the change of temperature, the amount of change in a Bragg wavelength is larger than the amount of change in a bandgap wavelength, with the result that the Bragg wavelength comes out of a wavelength range in which the Bragg wavelength exerts the gain.
Accordingly, it is an object of the present invention to provide a DFB semiconductor laser which has overcome the above mentioned problem of the prior art.
Another object of the present invention is to provide a DFB semiconductor laser oscillating in a stable single mode with a wide temperature range.
The above and other objects of the present invention are achieved in accordance with the present invention by a distributed feedback semiconductor laser having at least an active layer and a diffraction grating on a semiconductor substrate and so constructed that a current is injected uniformly over an axial direction of a resonator, wherein a bandgap wavelength of the active layer in the neighborhood of a light outputting end or a backward end opposite to the light outputting end, is shorter than in the other region of the active layer along the axial direction of the resonator.
According to a second aspect of the present invention, there is provided a distributed feedback semiconductor laser having at least an active layer and a diffraction grating on a semiconductor substrate and so constructed that a current is injected uniformly over an axial direction of a resonator, wherein a bandgap wavelength of the active layer both in the neighborhood of a light outputting end and in the neighborhood of a backward end opposite to the light outputting end, is shorter than that in the other region of the active layer along the axial direction of the resonator.
According to a third aspect of the present invention, there is provided a distributed feedback semiconductor laser having at least an active layer and a diffraction grating on a semiconductor substrate and so constructed that a current is injected uniformly over an axial direction of a resonator, wherein the active layer includes, along the axial direction of the resonator, a constant bandgap wavelength region and a changing region where a bandgap wavelength gradually shortens from the constant bandgap wavelength region towards a light outputting end and/or a backward end opposite to the light outputting end.
According to a fourth aspect of the present invention, there is provided a distributed feedback semiconductor laser having at least an active layer and a diffraction grating on a semiconductor substrate and so constructed that a current is injected uniformly over an axial direction of a resonator, wherein the active layer includes, along the axial direction of the resonator, a longest bandgap wavelength position and a changing region where a bandgap wavelength gradually shortens from the longest bandgap wavelength position towards a light outputting end and/or a backward end opposite to the light outputting end.
According to a fifth aspect of the present invention, there is provided a distributed feedback semiconductor laser having at least an active layer and a diffraction grating on a semiconductor substrate and so constructed that a current is injected uniformly over an axial direction of a resonator, comprising a window region at a light outputting end and a bandgap wavelength changing region where a bandgap wavelength of the active layer gradually changes along the axial direction of the resonator, the bandgap wavelength changing region being adjacent to the window region.
In at least a portion of the bandgap wavelength changing region, a modulation region is preferably provided in which the period of the diffraction grating is modulated with the change of the refractive index in the active layer.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.